Vacuum actuated test fixtures have been found to be highly effective for testing many types of printed circuit boards. Some of the heretofore available test fixtures of this type generally comprise a probe plate having a plurality of spring-loaded electrical probes thereon, and a casing supported above the probe plate and spaced away therefrom by a peripheral resiliently deformable seal. The casing includes a perforated top plate, the undersurface of which rests against the resilient seal and onto which a circuit board can be mounted. The probes of the probe plate can pass through the perforations of the top plate. The casing also includes a lid pivotally mounted to the upper surface of the top plate, which can be pivoted away from the top plate into an open position, and towards the top plate into a closed position. The lid comprises a peripheral lid frame and a lid plate fixedly attached to the top peripheral edge portion of the lid frame. Moreover, the lid plate comprises a number of downwardly projecting abutment fingers registering above the circuit board when the lid is closed. A peripheral resiliently deformable sealing member is attached to and depends downwardly from the lower rim of the lid frame, and rests against the top plate when the lid is closed.
To test a circuit board using such a test fixture, a circuit board is mounted on the top plate, the lid is set in its closed position with the lid's resilient sealing member sealing the inner chamber within the lid, and a vacuum is created between the top plate and the probe plate, and between the top plate and the lid. It is to be noted that when the lid is closed, but before a vacuum is created between the top plate and the lid, a substantial gap, typically of a few centimetres, separates the lid frame's lower rim and the top plate's upper surface, with the resilient sealing member filling this gap.
The creation of a vacuum between the top plate and the probe plate, and between the top plate and the lid, causes the following displacements to occur concomitantly:                the lid is drawn downwardly towards the top plate which movement is allowed by the sealing member resiliently yieldingly compressing to allow the lower rim of the lid frame to move towards the top plate's upper surface. As the lid is drawn towards the top plate, the abutment fingers projecting from the lid plate are moved downwardly in order to press the circuit board against the top plate, and thus secure the circuit board to the top plate; and        the top plate is drawn downwardly towards the probe plate in order for the probes mounted on the probe plate ands extending through the perforations made across the top plate to abut against the undersurface of the printed circuit board to electrically connect the probes with corresponding predetermined circuits of the circuit board. When the probes come in contact with the circuit board, the probes apply upward pressure on the circuit board but the circuit board remains still since it is held down by the abutment fingers.        
As useful as these test devices may be, their usage involve important security hazards. Indeed, it happens that an operator inadvertently places his finger(s) in the gap between the lower rim of the lid flame and the top plate, and then activates the vacuum pump to create a vacuum between the lid and the top plate. As the vacuum pump is activated, the lid is thrust downwardly against and crush the operator's misplaced finger(s), which can cause severe injuries.
Also, the above-mentioned prior art test devices also have some inaccuracy issues. Indeed, when the test fixture is made and assembled, there are often small undesirable plays that appear when the lid is installed on the top plate of the test fixture. This is at least partly due to the fact that the lid is designed to be pivotally movable relative to the top plate to allow access to the inner test chamber where the circuit board to be tested is installed, and consequently a high degree of precision is more difficult to achieve during the fabrication and assembly thereof. These undesirable plays may result in the lid slightly moving laterally when a vacuum is created therein, which results in turn in some lid-carried probes not engaging their corresponding contacts on the circuit board, among other problems related to this inaccuracy of the lid position.